1. Field of the Invention
The present invention relates to a memory for storing information, and more particularly, to a nonvolatile memory using a magneto-resistance effect element and an information recording and reproducing method therefor.
2. Related Background Art
Generally, magnetic substances, such as ferromagnetic substances and ferrimagnetic substances, have a characteristic that magnetization caused by magnetic fields applied from the outside resides even after the external magnetic fields are removed (this residing magnetization is called the xe2x80x9cresidual magnetizationxe2x80x9d). These substances are hereinafter collectively referred to as the xe2x80x9cmagnetic substancesxe2x80x9d. Also, the electrical resistances of these magnetic substances change according to the direction of magnetization and the presence or absence of magnetization. This phenomenon is called the xe2x80x9cmagneto-resistance effectxe2x80x9d. Also, the changing rate of an electrical resistance value is called the xe2x80x9cmagneto-resistance ratio (MR ratio)xe2x80x9d. As a material having a high MR ratio, there have been known giant magneto-resistance (GMR) materials and colossal magneto-resistance (CMR) materials that are metals, alloys, or oxide complexes. For instance, Fe, Ni, Co, Gd, Tb, alloys thereof, and LaxSr1-xMnO9 and LaxCaxcex-xMnO9 that are oxide complexes are used as the GMR materials and CMR materials. By utilizing the residual magnetization of the magneto-resistance materials, it is possible to construct a nonvolatile memory in which information is stored on the basis of a change in electrical resistance value caused by the direction of magnetization and the presence or absence of magnetization. This memory is called the xe2x80x9cMRAM (Magnetic Random Access Memory)xe2x80x9d and various researches thereon have been conducted in recent years.
Many of MRAMs that have been under development in recent years adopt a scheme where information is stored using the residual magnetization of a ferromagnetic substance made of a giant magneto-resistance material and the stored information is read by converting a change in electrical resistance value caused by a difference of magnetization directions into a voltage. Also, information is written into a ferromagnetic memory cell and the information is rewritten by changing the magnetization direction in the ferromagnetic memory cell using a magnetic field induced by current flowing through a write line.
The cell structure of an MRAM and the driving method therefor is described by R. E. Scheuerlein, xe2x80x9c1998 Proc. of Int NonVolatile Memory Conf. P47xe2x80x9d. This document proposes a structure in which a pair of write lines is arranged perpendicular to a pair of read lines. This document also proposes a structure (of a matrix type) in which a memory cell is connected to a diode in series and includes a giant magneto-resistance thin film that forms a pair of wires arranged perpendicular to each other and functions as both of a write line and a read line.
Also, U.S. Pat. No. 5,448,515 discloses a memory cell of a 1T1R type (meaning that each unit cell includes one transistor and one magneto-resistance element) in which a pair of lines arranged perpendicular to each other functions as both of a write line and a read line and a field effect type transistor used for cell selection is combined with a resistant element including a giant magneto-resistance thin film. This memory cell including the giant magneto-resistance thin film exhibits a magneto-resistance effect where different electrical resistance values are obtained by changing the direction of magnetization. This patent document also discloses a method of reading information stored in the 1T1R type memory cell.
Further, a document xe2x80x9c2000 Proc. of Int Solid-State Circuits Conf. P128xe2x80x9d proposes a memory cell having a 2T2R type structure where two field effect type transistors are combined with two TMR devices. With this structure, it is possible to increase signal strengths by complementarily setting the resistance values of the two TMR devices.
In the case of the matrix type structure proposed by R. E. Scheuerlein in xe2x80x9c1998 Proc. of Int NonVolatile Memory Conf. P47xe2x80x9d, the strength of each signal is low and therefore it is difficult to detect information with stability.
In the case of the memory cell of the 1T1R type disclosed in U.S. Pat. No. 5,448,515, even if a tunnel magnetoresistance (TMR) element exhibiting a relatively high magneto-resistance change rate is used, the change rate of a resistance value caused by the application of a voltage of around 0.3 V is in a range of from 20% to 30% at best. If the applied voltage is further increased, there occurs a steep decrease in the magneto-resistance change rate, so that it becomes difficult to read information properly. Also, an extremely high MR ratio is required to curb the influences of fluctuations of resistance of the TMR element, the fluctuations of on-resistance of the transistor, and the like.
Further, in the case of the 2T2R type structure proposed in xe2x80x9c2000 Proc. of Int Solid-State Circuits Conf. P128xe2x80x9d, a cell area becomes around twice as large as that in the case of the 1T1R type.
As described above, it is difficult to produce a 1T1R type MRAM, whose cell area is small and which operates with stability, using a variable resistor whose magneto-resistance value is changed by selecting the magnetization direction of a magnetic substance.
The present invention has been made in light of the above-mentioned problems with the conventional techniques, and therefore has an object to provide a memory structure enabling both of the reduction of the cell area of an MRAM and the stable detection of information stored in the MRAM, and to provide a driving method therefor.
The above-mentioned object is achieved by an information reproducing method for a magnetic memory including a variable resistor having a hard layer that is made of a magnetic substance and stores information on the basis of a direction of magnetization, a non-magnetic layer, and a soft layer that is made of a magnetic substance and has a coercive force that is smaller than that of the hard layer, the information reproducing method comprising the steps of: initializing the soft layer; detecting and holding a resistance value of the variable resistor; detecting another resistance value by reversing magnetization of the soft layer; and reproducing information by comparing the held resistance value with the resistance value detected afterward.
With this information reproducing method, the magnetization direction of the soft layer is reversed in order to change the resistance value of the variable resistor and information stored in the hard layer is read on the basis of a difference between the held resistance value obtained before the reversal and the resistance value obtained after the reversal. This makes it possible to read the stored information with precision in a state where the information remains in the hard layer. As a result, it becomes unnecessary to rewrite the information into the hard layer after the reading operation.
Also, it is more preferable that an insulator is used as the non-magnetic layer because it becomes possible to increase a magneto-resistance change rate.
Further, it is more preferable that each magnetic layer has magnetization in a vertical direction with reference to a film plane because it becomes possible to miniaturize a memory cell.
The above-mentioned object is also achieved by a magnetic memory comprising: a variable resistor including a hard magnetic layer for storing information on the basis of a direction of magnetization, a non-magnetic layer, and a soft magnetic layer having a coercive force that is smaller than a coercive force of the hard layer; a magnetic field generating means for initializing magnetization of the soft layer and reversing the initialized magnetization of the soft layer; a storage circuit of holding a resistance value obtained in a initialized state; and a signal detecting circuit for outputting a reproduction signal by comparing the held resistance value with a resistance value of the variable resistor obtained after the reversal.
With this structure, the storage circuit holds the resistance value of the variable resistor in a state where the soft layer is initialized by the magnetic field generating means, the resistance value of the variable resistor is changed by reversing the magnetization direction of the soft layer using the magnetic field generating means, and the signal detecting circuit reads information stored in the hard layer on the basis of the resistance values before and after the reversal. This makes it possible to read the stored information with precision in a state where the information remains in the hard layer. As a result, it becomes unnecessary to rewrite the information into the hard layer after the reading operation.
Here, the magnetic memory may further comprise: a plurality of bit lines that are parallel to each other; a plurality of word lines that are parallel to each other and are perpendicular to the bit lines; and a switching element connected to a word line formed on a semiconductor substrate so as to pass near a control terminal, one of terminals of the switching element being grounded, wherein the variable resistor and the switching element are provided for each intersection of the bit lines and the word lines, and one of terminals of the variable resistor is connected to the other of the terminals of the switching element, and the other of the terminals of the variable resistor is connected to a nearby bit line.
Also, the storage circuit may be a capacitor that is provided between a ground potential and one of two input terminals of a sense amplifier into which the potential of the bit line in the initialized state is inputted.
This structure is more preferable because it becomes possible to stabilize inputs into the sense amplifier using the capacitor, and to read information with more precision.
Here, another capacitor may further be provided between the ground potential and the other of the input terminals of the sense amplifier.
This structure is more preferable because it becomes possible to stabilize inputs into the other of the input terminals of the sense amplifier using the capacitor and to read information with more precision.
According to one aspect of the present invention, the switching element is a field effect type transistor, with a gate terminal being used as the control terminal, a source terminal being used as the terminal of the switching element that is grounded, and a drain terminal being used as the terminal of the switching element that is connected to a terminal of the variable resistor.
In addition, it is preferable that an insulator is used as the non-magnetic layer because it becomes possible to increase a magneto-resistance change rate.
Furthermore, according to another aspect of the present invention, it is preferable that each magnetic layer has magnetization in a vertical direction with reference to a film plane because it becomes possible to miniaturize a memory cell.
The present invention will be described in more detail in accordance with the following embodiments.